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No info for a smaller concrete job


Captain Ledd

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First off, I've never worked with concrete except as a kid helping my dad set the swingset in, and some posts under our back porch steps.


I can't find much information on multiple pours of concrete. The consensus is that one monolithic pour is best. But we need 10-11 cu/ft per slab and we're not sure if we can rent a mixer that large. How long do we have to make another pour of concrete? Can we do that much in a wheel barrow? (assuming we don't really stop). Is it easier with a smaller rentable mixer? Everything I've looked at is for house footings and walls and laaarge amounts/expanses of concrete. I just want to know how much time I have between each pour so it doesn't crack all to shit and fall over.


Back story:
Me and my dad recently went in on a Rotary SPOA9 2-post lift :yahoo::headbang: (asymmetric lift, so a large part of the vehicle is off to one end). The problem is to install it is that the posts sit right on the cracks for the 3 slabs of concrete in the Quonset hut, so we need to pour some new chunks of concrete. Each of the 3 slabs are 10' wide and 60' long, each have only one crack in them. It was built in the early 60's.

We're going with a 8" thick, 4'x4' slab (might as well go a bit thicker right?, also we don't know how thick the current slabs are or what psi rating and need to make sure it's keyed under or fairly self sustainable at least). Which should be around 10-11 cu/ft.

I just want to make sure that despite my amateur-ness this thing doesn't fall over on us.

ANY tips or information would be appreciated.
 


shane96ranger

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I'm no expert, but I can tell from my experience last July that you don't want to go much beyond an hour if you are adding it to a pour. My BIL told his guys to order 9.5 yards, but his guy measured and said it only needed 8.5 - so that's what he ordered. Well, turns out they really needed 9.5 yards - and so it sat for an hour while another truck delivered the additional yard. It went together OK, but I can tell where the two different trucks came.

On a side note, I have no cracks anywhere - yet. I had them add the fiberglass fibers and also bought the 4500 psi concrete instead of 3000 psi (my neighbor did a 3000 psi pour the weekend before, and his has several cracks already). I thought the heavier duty concrete may come in handy if I decide to park something massive on it.
 

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In my experience you won't want to mix more than two bags at a time in a wheelbarrow. Make sure you use enough re-bar. In my opinion you'd be better off ordering from a ready mix company and they can help figure out how much psi to use.
 

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I drove ready mix truck for 28 yrs, 4000lb mix is great, fibers help, so does re bar.
if you have a 10x60 ft slabm unless ya got help or else it will flash on ya.
if its just the 4x4 1/2 way up, add some rebar horizontal and verticle, helps in the long run.
dont use too much water in your mix or the gravel will float down and lots more shrinkage. just spray a lil water on the edges when you make another batch, then use shovel to mix it all together at the edges.take your time, itll work out
 

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Keep it wet between pours and when it's complete, keep it covered for a few days to keep it damp.
 

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I rented a mixer from Home Depot a few years back. I had extended one of the porches to add a sun room. I added about 4 yards. You don't want to do that in a wheel barrow. The mixer was worth every minute of time. Made it so much easier. Now you can get a concrete truck to come to the house and they can pump it where ever you want them to direct the hose. The truck can stay out front and the hose can go anywhere. So cool and so easy. The hardest thing to do is the framing for the concrete. They have trucks that come to your home and they do mulch the same way....they just use a hose attachment and have a blower that sucks up the mulch in the truck and spreads the mulch in the area it is going. So cool and cost effective.
 

Captain Ledd

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I drove ready mix truck for 28 yrs, 4000lb mix is great, fibers help, so does re bar.
if you have a 10x60 ft slabm unless ya got help or else it will flash on ya.
if its just the 4x4 1/2 way up, add some rebar horizontal and verticle, helps in the long run.
dont use too much water in your mix or the gravel will float down and lots more shrinkage. just spray a lil water on the edges when you make another batch, then use shovel to mix it all together at the edges.take your time, itll work out
The 10x60 is what's there now, we're chopping out a 4'x4' section and re-pouring a solid slab. The lift posts sit right on the seam of the existing slabs. Manual also states it should be keyed under the existing slab, makes sense.

I just don't think we have enough volume to warrant an entire cement truck. We could do a large slab in front of the door, but really it'd be a "nice" thing, and completely unnecessary. All the rental ones (that they list) won't do what we need in one pour.

Thanks for you input Kroutster!

I know we need minimum 3000 psi concrete. We were looking at 4000-5000psi stuff. I was going to add some rebar, but I have no idea if I'd be using it right, I like the idea of laying it down 1/2 way through the pour. We're overbuilding it because of 1) just to be that much safer and 2)we have no idea what the existing concrete is.

*edit: Minimum specs from Rotary are: 4'x4' wide, 4" thick, 3000 psi or better, keyed under existing slab.
 
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Damon Toth

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Use concrete from a ready mis supplier- order 5000 psi mix 3/8" pea gravel pump mix with stealth fibers.
The concrete is only as good as the sub-grade; remove the existing concrete and soil at the locations of the new foundation pads and check the soil conditions. if the soil is wet, or loose, remove the soil, compact the bottom and replace the section removed with crushed aggregate back up to bottom of foundation.
The rebar should be a double mat, (4) #5 each way top and bottom.
You may want to drill and epoxy dowels into the existing concrete to tie the foundations together.
It would help to know what kind of loads will be placed on each foundation.
 

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I've done a lot of jobs with concrete lately. I mixed 24 bags today, as a matter of fact.

If you buy 60# bags they are good for 1/2 cubic foot. You need a garden hoe, a small bucket with graduations marked in it and a wheel barrow. Do one bag at a time because it makes it much easier and faster to mix the water in. Do 3 quarts of water per bag. Drop the bag into the wheel barrow and split it open with the hoe. Pull the bag free and pour the water into the barrow on top of the concrete mix. Use the hoe to mix it up--you'll get where you can mix it well in a couple of minutes. Dump or shovel it out and start over. It takes a couple of hours to do 10-11 cubic feet. That's no problem at all. You can do 2 bags in the wheel barrow but the mixing time is much higher and it is quicker to do one at a time.

The water mix is critical to time. Use 3 quarts for 60# and measure. It will mix up with the perfect slump. If it is a little wet, fill the bucket a little under the 3qt line. etc. Don't guess.
 

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Ok.... as someone who worked concrete construction for over 10 years for a VERY reputable local company (who also had far less than the industry norms for failure and cracks), I guess I should weigh in...

You'll want to go with a redi-mix. No two ways about it. Yes, it can be mixed by hand or in a small gas/electric mixer, but without the proper testing equipment, there is no way to tell just how good of quality stuff is coming out of it. Sure, it will hold up just fine for replacing part of a sidewalk or patio or the like, but are you really going to feel safe when you have a 6,000 lb truck hanging on a two post lift above you while you yank on a breaker bar with a cheater pipe on a stubborn bolt?

Bagged sackcrete materials give arbitrary compression strength numbers. That's if you follow the instructions EXACTLY, cross your fingers and hope for the best that you may actually achieve those numbers. Redi-mix stuff is DESIGNED by engineers to meet the minimum strength provided you pour and cure it correctly (which usually means a 4" slump or less, finish it AFTER the bleed water evaporates, and cure it according to ACI (American Concrete Institute) specs - which for interior work means you don't try to make it cure faster).

IMHO, if it says a min of 4' square, 4" thick and 3,000 psi, I would do at least a 5' square, 6" thick and 4,000 psi min, just because I like overkill. But to key it under an existing slab, you will have to make it thicker and use TWO layers of reinforcement. But... depending on the thickness of the EXISTING slab, this may be difficult. Say the existing slab is 4" thick, you will have to pour 8" thick with a layer of 1/2" rebar in the first 4" and a layer in the top 4". If the existing slab is more than 4" thick, I would NOT recommend keying the new slab under the existing slab but rather to make the new slabs larger. Thicker is not always better.

I would also put AT LEAST 2-3" of crushed stone (around here we use limestone) under the new slabs and compact it with a hand tamper or jumping jack (aka trench tamper). This is to provide a good base and to allow drainage for any water seepage.

I would recommend lining the hole with at least 1/4" thick expansion joint (vinyl, rubber or asphalt - personally I'd use vinyl).

A pair of 5' square slabs 6" thick works out to be just under 1 yard of concrete (the minimum most redi-mix companies will deliver and if it was me, I would order at least a yard and a quarter - there is always the chance of losses for various reasons. You will want to tell them that it is for a basement floor slab as well (this determines the amount of entrained air they use which accounts for both durability and workability).

It is expensive getting a concrete mixer, but believe me, it is a better quality mix (most of the time anyway - some redi-mix companies are better than others). Locally it is around $150-200 for one to two yards delivered (they whack you pretty hard for short load charges) but under some circumstances, it's well worth the money. A couple years ago I had a job that needed done late in the year and needed to be durable quickly, so I bit the bullet and got redi-mix even though it was only a yard that I needed. I also got high-early strength cement in the mix so it would gain strength and cure faster... worked out perfectly - by 4pm I was in the local bar warming up from the cold. Had I tried to do that with sackcrete, I would have still been waiting on it to get to a point that I could finish it - and it probably would have self-destructed after the temps dipped to the low 20's that night.
 

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According to my son, foreman for a flatwork crew, you just nailed it lil_Blue_Ford!
 

shane96ranger

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I rented a mixer from Home Depot a few years back. I had extended one of the porches to add a sun room. I added about 4 yards. You don't want to do that in a wheel barrow. The mixer was worth every minute of time. Made it so much easier. Now you can get a concrete truck to come to the house and they can pump it where ever you want them to direct the hose. The truck can stay out front and the hose can go anywhere. So cool and so easy. The hardest thing to do is the framing for the concrete. They have trucks that come to your home and they do mulch the same way....they just use a hose attachment and have a blower that sucks up the mulch in the truck and spreads the mulch in the area it is going. So cool and cost effective.
Getting a pump is pretty expensive here. Seems to me it was going to add around 400 dollars if we would have brought one in. My BIL just trucked it in my back yard with a skid steer, which was relatively quick. It was only necessary for about 3 yards, and the truck did the rest.
 

Captain Ledd

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Locally it is around $150-200 for one to two yards delivered (they whack you pretty hard for short load charges) but under some circumstances, it's well worth the money
Shoot, even at $200 that's nearly worth it. I thought it would run near $1000 to bring a truck out.

I'll give the local ones a call and see what they actually charge. Maybe I'll frame out that patio in front of the doors after all.

Meanwhile, borrowed a rotary hammer and the concrete is only 3" thick! I honestly thought it was heavier duty. We've even had a complete engine block fall on it and it only left a little dent.

The area is marked out (we're going 4'x4') and we're renting the saw tomorrow.

Thank you gentlemen, you have reassured me I can make this work one way or another.
 

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Now since your pouring mostly footing pads, you might be alright with the mixer, but I think you'd be better off getting a truck to come in if you're pouring that many yards. My dad and I extended my back patio last fall, with a perimeter footing, and a simple rectangular 8'x8' pad. I ordered redi mix. The truck was able to boom into my backyard about halfway from the property line to the patio, and we wheel barrowed back and forth (maybe a 12' path).

He's a superintendent for a construction company (going on 40 years of doing this), and it was just me and him and intermittent help from an older gentleman (neighbor) and 6 yds of concrete. And we barely made it. When he was finishing it out, it was starting to get too far set up. We maxed out what 2 guys could do. Mixing that much concrete in a mixer might not give you the results you want...though I guess you have the luxury of doing it in two jobs 1 for each footing, and you might be alright as you say it would be about 5.5yds in a concentrated area that you might be able to keep up.

For me it was closer to $100/cu. yd. here in KS (near KC). It was a little over. I just remember all the form work and materials and new tools I had to buy was close to $300, and the total was around $1000. So I'm pretty confident it was $100/cu. yd and some delivery surcharges and a fuel surcharge.
 
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shane96ranger

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Shoot, even at $200 that's nearly worth it. I thought it would run near $1000 to bring a truck out.

I'll give the local ones a call and see what they actually charge. Maybe I'll frame out that patio in front of the doors after all.

Meanwhile, borrowed a rotary hammer and the concrete is only 3" thick! I honestly thought it was heavier duty. We've even had a complete engine block fall on it and it only left a little dent.

The area is marked out (we're going 4'x4') and we're renting the saw tomorrow.

Thank you gentlemen, you have reassured me I can make this work one way or another.
I paid 1300 for 9.5 yards, with 2 delivery charges
 

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